Controlled solidification of case structures by controlled circulating flow of molten metal in the solidifying ingot

ABSTRACT

Continuously casting an ingot wherein the ingot has a pool of molten metal extending downstream a substantial distance from the casting mold, entrained by an outer skin of the ingot formed during passage through the mold. The molten metal within the pool is continuously circulated along the solid/liquid interface advantageously for at least a major portion thereof in a first direction the same as that of ingot movement and is returned in an opposite direction within the interior of the pool. This sweeping flow of metal along the interface changes the solidification microstructure from that which would be normally present, and results in the formation of improved and advantageously new products, e.g., new steel bodies, particularly noted by unique microstructures. The flow rate is chosen to provide a variety of novel solidification structures, i.e., modified equiaxed dendritic structure, or structures herein described as thamnitic of fibrous structures, within the ingot. The circulation of the molten metal may be provided by a helical coil which extends about the ingot downstream from the mold and, in the optimum arrangement, for substantially the entire extent of the molten pool within the ingot.

[ 51 Sept. 26, 1972 United States Patent Tzavaras [54] CONTROLLED SOLIDIFICATION OF CASE STRUCTURES BY CONTROLLED Primary Examiner-R. Spencer Annear CIRCULATING FLOW OF MOLTEN Attorney-Robert P. Wright and Joseph W. Maileck METAL IN THE SOLIDIFYING INGOT [72] Inventor:

Alexander A. Tzavaras, Broadview ABSTRACT Heights, Ohio Continuously casting an ingot wherein the ingot has a [73] A i Repubnc Sted Corporation Cleve pool of molten metal extending downstream a substanand Ohio tial distance from the casting mold, entrained by an outer skin of the ingot formed during passage through Filed! g- 1970 the mold. The molten metal within the pool is continu- [211 App] No: 65 611 ously circulated along the solid/liquid interface advantageously for at least a major portion thereof in a first direction the same as that of ingot movement and is returned in an opposite direction within the interior of the pool. This sweeping flow of metal along the interface changes the solidification microstructure from that which would be normally present, and results in the formation of improved and advantageously new [56] References C'ted products, e.g., new steel bodies, particularly noted by UNITED STATES PATENTS unique microstructures. The flow rate is chosen to provide a variety of novel solidification structures, i.e.,

2,877,525 3/1959 Schaaber.........;........... 164/49 modified equiaxed dendrific structure or structures 3,354,935 il/1967 Mann.......................164/82 X herein described as thamnitic f fibrous structures 3,517,726 6/1970 M1115 et x within the ingot. The circulation of the molten metal FOREIGN PATENTS 0R APPLICATIONS may be provided by a helical coil which extends about the ingot downstream from the mold and, in the op- 526,455 6/1956 Canada........................164/49 timum arrangement, for Substantially the entire extent 531,772 10/1956 Canada......................164/283 of the molten poo] within the ingot 783,427 9/1957 Great Britain...............164/82 173,173 11/1960 Sweden .......................164/49 15 Claims, 15 Drawing Figures PATENTEDsms I972 SHEET 1 or 6 INVENTOR. ALEXANDER (4. 72A VAR/4S BY M J? @AJ ATTOEA/EV PATENTEBSEPz I912 Q 3.693.697

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CONTROLLED SOLIDIFICATION OF CASE STRUCTURES BY CONTROLLED CIRCULATING FLOW OF MOLTEN META'L IN THE SOLIDIFYING INGOT BACKGROUND OF THE INVENTION This invention relates to casting methods and ap paratus, and more particularly to methods and apparatus for improving in a continuous casting process what has generally been termed in the past the grain structure of an ingot.

Much attention has been focused in the past on the improvement of grain structure in castings. It has been known that discontinuities, such as segregation, porosity and inclusions which affect ductility and reduce desirable characteristics, are related to dendritic structures in casting. Thus in the past grain refinement in many cases actually referred to dendrite refinement.

Principles of Solidification by Bruce Chalmers (John Wiley & Sons, 1964) gives a complete discussion of dendrite structures and characteristics in castings. As noted, it has been recognized that such structures may adversely affect the properties of a casting such as an ingot. Schaaber U.S. Pat. No. 2,877,525, issued Mar. 17, 1959, is directed to the improvement of cast structures utilizing rotation of molten metal about the longitudinal axis of an ingot during the casting process. Certain of the disadvantages of rotation are that it contributes to centerline porosity or shrinkage and that it does nothing to transport the elements of the freezing zone back into the molten metal so that solute concentrations and inclusions are taken out of contact with the freezing front and thereby do not become entrapped.

. Further, molten metal rotation aggravates problems of segregation inasmuch as the heavier elements in the molten metal tend to be moved by centrifugal force to the outer zone of molten metal movement. Still further, rotation is undesirable because it segregates inclusions at the center of the ingot and prevents them from rising to the surface.

Criner U.S. Pat. No. 3,153,820, issued Oct. 27, 1964, discloses the stirring of molten metal during ingot fortning through the use of toroidal coils about a mold which induce eddy currents in the melt in order to stir or vibrate the melt. Although vibration may overcome the problems of centerline porosity or shrinkage, inclusions within the melt are not brought to the surface in contact with the slag layer so as to be removed. Further the positioning of stirring coils about the chilling mold is generally not a good location for such coils because of the tendency to disturb the initial skin which is forming on the outer surface of the ingot.

British Pat. No. 752,271 purports'to disclose stirring of molten metal in the casting of an ingot through the use of an induction coil positioned adjacent to the mold. Metal movement toward and away from the upper surface of the molten metal is disclosed. The disadvantage of proximity of stirring coil to mold is present in this arrangement.

A similar movement of metal adjacent induction coils is shown in British Pat. No. 705,762, for the purpose of achieving rapid movement of molten metal through the outlet of a molten metal container and into the chilling mold for the purpose of ingot formation.

BRIEF DESCRIPTION OF THE INVENTION The present invention is directly concerned with casting procedures, and involves microstructure refinement. It has particular application to the continuous casting of an ingot in which the pool of molten metal inside the ingot extends a substantial distance below the chill mold or other mechanism used to form the initial outer skin of the ingot. Such a molten pool may extend, for example, 30 and upwards or even as much as feet below the chill mold in the case of a high speed continuous casting of an ingot.

It has been known that the important parameters which determine the mode of growth for a cast structure are the temperature gradient in the molten metal along the solid/liquid interface, the growth rate of solid material, and the solute concentration in the liquid. In the present invention, a flow of molten metal is utilized substantially to increase the temperature gradient. The typical mushy zone that is normally present along the solid/liquid interface is influenced, greatly reduced or in effect eliminated, changing the solidification characteristics of the ingot so as to produce a more desirable ingot structure. The flow is such as to prevent the growth of columnar dendrites which are known to be undesirable in ingot structure.

The fiow of metal that is utilized is a sweeping flow which is along the solid/liquid interface for a selected length thereof or in many cases desirably for substantially the full extent thereof in a first direction that is advantageously the same direction as that of ingot movement; a reverse path of circulation can be used in some cases, but requires substantially greater power for electromagnetically advancing the metal because it must then be moved oppositely to the normal convection flow. A return flow in an opposite direction within the interior of the molten pool is utilized to provide for complete and effective circulation of all the metal within the pool. This sweeping fiow of metal brings hot metal from the top of the molten pool along the solid/liquid interface washing away or substantially dissolving the mushy zone and breaking dendrites before they can form into longer columnar dendrites. The return flow circulates broken dendrites back into the hot melt where they are either re-melted or carried around to be deposited in random orientation and in different locations from where they were broken. The return flow also sweeps the molten metal into contact with the underside of the slag layer, by which many inclusions are easily removed. The downward sweeping flow of hot metal along the interface also tends to redissolve inclusion elements, even to the extent of preventing their deposit as inclusions until solidification of the trailing end of a continuously cast strand. The metal circulation further promotes coagulation of small inclusions, and aids their separation, as larger bodies, into the slag layer.

The sweeping flow which takes place in accordance with the invention and which moves in the interior of the molten pool reduces segregation and centerline porosity or shrinkage that is present when rotation about the ingot axis is employed as in prior art stirring techniques during ingot formation. As noted, the sweeping flow may advantageously be for the full extent of the molten pool and thus extends many feet downstream of the chill mold or other chilling means. Advantageously the sweeping flow of molten metal is achieved through the use of helical coils positioned along the ingot downstream of the chill mold. Optimum flow of molten metal is believed to be achieved when the effect of the coils covers the full length of the mo]- ten pool. Reference to a helical coil thus means a coil in which the turns form a helix around the ingot or strand, i.e., a helix (whether having square or oblong turns, with rounded corners, or circular turns or other shape suited to the ingot) that is substantially coaxial with the ingot and its path of travel. The coils are energized in known fashion by a polyphase source of excitation to achieve the desired circulation of molten metal.

Depending upon the flow rate, different growth structures within the ingot are produced. Generally with laminar flow the dendrites are broken so that columnar dendrites cannot result and an equiaxed dendritic structure (random orientation of dendrites) results. At higher fiow rates, what I term thamnitic or flow-modified structure develops which involves round shapes and in which arms, either primary or secondary, cannot be distinguished, as they can be in typical dendrite structures. At still higher flow rates what I term fibrous growth results (similar to cellular) characterized by unidirectionally aligned primary structural elements without any secondary structural elements or secondary arms.

Each of these flow rates is characterized by a growth structure which is different from that which would be present under the same thermodynamic conditions without any forced fluid flow.

Accordingly, the invention has for a primary object the achieving of improved solidification microstructures.

The invention will be more completely understood by reference to the following detailed description taken in conjunction with the appended drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of representative apparatus in accordance with the invention, shown in greatly simplified and somewhat schematic manner.

FIG. 2 is a sectional view taken along the section 2- 2 of FIG. 1.

FIGS. 3a, 3b, 4a, 4b, 5a, and 5b are scanning electron microscope photographs of difierent microstructures in accordance with the invention.

FIG. 6 is a series of curves showing the relationship between different growth structures as a function of solute concentration, temperature gradient and growth rate.

FIG. 7 is a series of curves relating the growth structure to initial temperature and flow rate or power input.

FIG. 8 is a micrograph (magnification 65x) of a section taken parallel to the direction of growth (i.e., growth from a chilled surface), showing a broken columnar microstructure, of equiaxed character.

FIG. 9 is a micrograph (magnification 65x) of a section parallel to growth, showing a thamnitic microstructure.

FIGS. 10a and 10b are micrographs (magnification 65x, reduced for reproduction) of sections respectively taken parallel and perpendicular to the direction of growth, showing a fibrous microstructure.

FIG. 11 is a micrograph (magnification 65X, reduced for reproduction) of a section parallel to growth, which proceeded as from left to right, showing dendritic microstructure except for a band across the growing solid where the molten metal was subjected to flow as in accordance with the invention, there producing a thamnitic microstructure.

DETAILED DESCRIPTION Before the figures are described in detail, it is best to consider dendrite growth. Dendrite growth is characterized by a resultant microstructure which is somewhat tree-like in appearance, having primary and secondary arms. The primary arm of a dendrite extends in one direction similar to the trunk of a tree, and the secondary arms extend generally perpendicular thereto as do the branches of a tree. The secondary arms themselves may have further arms extending perpendicular thereto. Columnar dendrite structures are characterized by relatively long primary dendrite arms which align themselves substantially parallel to each other. Columnar dendritic structures are undesirable in a cast structure since at least the ductility varies according to direction; it is strongest in the growth direction of the primary dendrite arms and is poor across the growth direction. As noted above, rotational stirring about the ingot axis has been employed in the past to affect the growth structure and to eliminate columnar dendrites. However, such rotation aggravates centerline porosity (because of the centrifugalforces tending to move molten metal outwardly from the center of the molten pool) and is also undesirable because circular flow does not adapt itself to castings of square cross-section.

In achieving dendrite refinement in a cast structure, it is desirable to eliminate inclusions and segregation as much as possible. Inclusions formed prior to solidification are mainly oxides stable at high temperatures. Inclusions formed during solidification are most sulfides, tellurides, arsenides, nitrides and some oxides. The usual inclusions in steel are compounds of various solutes or deoxidizers used in steel combined with oxygen, sulfur, and less frequently with nitrogen. If rotary stirring about an ingot axis or local agitation is employed, inclusions, if affected at all, will still remain throughout the structure, or will tend to concentrate at the center during the solidification process. What is needed is a movement of metal which is past the slag layer so that the inclusions will be removed when the slag layer is contacted. Segregation is characterized by concentrations of elements at zones within the ingot and is aggravated in particular by rotary stirring, which tends to create segregation bands about the ingot axis because of centrifugal force. Macrosegregation is usually associated with large castings and refers to local changes in the solute concentration in a macroscale. Microsegregation is the difference in solute concentration between the center of the dendrite and the interdendritic areas. It will be understood that references to solutes mean, for example in steel, other elements than iron, such as alloying elements, contained in the desired composition.

All of these problems are obviated in the present invention by the use of a stirring technique as shown in FIG. 2. Referring now to FIGS. 1 and 2, molten metal to be continuously cast into an ingot, for example, is

supplied to a chill mold through a supply pipe 12 extending downward from a conventional tundish (not shown). The pipe includes an outlet 120 located within the molten pool and below slag layer 13, and orientated upwardly so that the molten metal exiting from the supply pipe is directed upwardly to a locality slightly below the intersection of the slag layer and the mold wall, as shown by arrows 14. The chill mold is supplied with a coolant via a supply line 16, which coolant circu-- lates within the interior l0a of the mold and exits therefrom via outlet 18. The chill mold is conventional in construction and is typically 2 or 3 feet long in the direction of the ingot movement, which is downwardly in FIG. 2. An ingot 20 is formed containing a pool 22 of molten metal therein. The bottom of the ingot is generally supported by a support structure 24 and by rollers 26 along the sides thereof. As shown in FIG. 1, the ingot, i.e., the cast strand, is square, although this is simply representative. lnterposed between the rollers 26 are a series of helically wound coil sections 28a, 28b, 28c, 28d, 28e, 28f 28n. Water for cooling purposes may be supplied as shown around the coil sections and rollers. The coil sections 28 are energized by a source of alternating current potential (not shown) such that the excitation of the sections varies in phase in a predetermined manner as will be readily understood. Any number of phases may be employed for the excitation. What is desired is a moving magnetic field which causes a flow of metal as shown by arrows 30 in FIG. 2.

In this regard electromagnetic stirring principles are well known. Electromagnetic stirring has been employed in connection with the melting and refining of molten metal. See for example the following references which deal with such electromagnetic stirring: Williamson U.S. Pat. No. Re. 24,463, issued Apr. 22, I958; Dreyfus U.S. Pat. No. Re. 24,462, issued Apr. 22, 1958; Dreyfus U.S. Pat. No. 2,774,803, issued Dec. 18, 1956; Tostmann U.S. Pat. No. 2,968,685, issued Jan. 17, 1961; I-Iokanson U.S. Pat. No. 3,239,204, issued Mar. 8, 1966; Jones U.S. Pat. No. 2,686,823, issued Aug. 17, 1954 and Magnetic Traveling Fields for Metallurgical Processes by Yngve Sundberg (IEEE Spectrum, May 1969, pages 79 through 88).

The stirring of molten metal by electromagnetic means is, as just noted, well known. The stirring mechanism involves the development of eddy currents within the molten metal by the varying magnetic field, which eddy currents themselves set up magnetic fields which interact with the applied magnetic field to cause movement of the molten metal. By using polyphase excitation of the coil sections 28, pulses of motive force are given to the molten metal progressively from section to section in the desired direction, so that the metal is caused to flow continuously downward along its outermost regions, in effect parallel to the axis of the coils. Thus the flow of molten metal may be as shown by the arrows 30, namely, sweeping downwardly over the solid/liquid interface of the ingot, as indicated by arrows 30a and 30b and upwardly through the center of the molten pool, as indicated by arrows 30c and 30d. The flow along the solid/liquid interface increases the temperature gradient in the liquid at the interface and reduces or eliminates the extent of the mushy zone normally present in a solidifying ingot at the interface,

all dependent upon the flow rate of the liquid in the molten pool. The flow rate is a function of the viscosity of the molten metal and the power applied to the coil sections 28. By appropriate reversal of the phases of excitation of the coil sections, an opposite flow could be achieved with respect to that shown in FIG. 2, namely, upwardly along the solid/liquid interface and downwardly in the center of the molten pool. However, the flow in FIG. 2 is to be greatly desired, namely, downwardly along the solid/liquid interface and upwardly in the interior of the molten pool, inasmuch as this flow is the same as and aids the normal convection flow that occurs in an ingot during solidification.

It will be noted that the molten pool 22 extends a substantial distance below the chill mold l0, and this distance may be as great as feet in some cases, depending upon the speed of ingot formation and movement. The bottom portion 22a of the molten pool is rounded or even flattened and not pointed as when no flow is present. This flattening effect is produced by the circulation of hot molten metal from the top of the pool along the sides of the pool to and across the bottom of the pool. As a result, the shape of the molten pool is frustoconical and not conical with a sharply pointed bottom as when no forced flow is present. It is believed that the volume of the force flow molten pool is the same as that of the non-forced flow molten pool. Hence when employing forced fluid flow, the length of the molten pool is much less than that present with no forced flow. One advantage of a shorter molten pool is that at the end of a continuous casting process, when the feed of molten metal to the ingot, i.e., the strand, is stopped, the wasted end length is shorter than that which would normally be present. A further advantage is that when successive lengths are cut from the continuously cast strand, there is much less danger that the plane of cut will intersect a pointed end of the liquid pool and allow molten metal to escape.

By placing the coil sections 28a below or downstream of the chill mold 10, the electromagnetic field will not be present to a substantial degree within the chill mold. This is helpful because it is desired to reduce turbulence and agitation within the mold at the location where the outer skin of the ingot is initially formed. The upwardly directed outlet 12a from the supply pipe 12 aids in the flow of metal in this regard, causing a full sweep of liquid on the underside of the slag layer so as to remove inclusions. The flow from the nozzle is directed generally toward or just below the intersection of the slag layer with the sides of the mold so as to avoid any turbulence in the slag layer.

Although specific helically wound coil sections have been shown, it is possible to use other helically wound coil arrangements. In particular, the coil may be continuously wound and may include taps at various points for connection to the various phases of excitation. Alternatively, a plurality of conductor strands may be wound helically about the ingot adjacent each other, each strand carrying an individual phase of excitation. Many alternatives will present themselves to those skilled in the art. What is desired, of course, is a fluid flow as discussed above. Further, the electromagnetic coils may be made of copper tubing, carrying a coolant for the purpose of heat dissipation from the electromagnetic structure. The effect of the electromagnetic structure should cover at least some part, and for special advantage, substantially the full length of the liquid pool in the downstream direction below the mold, the latter condition being desirable to achieve the full benefits of stirring of the pool of molten metal.

The amount of current carried in the coils will vary depending upon the degree of stirring desired and the material forming the molten pool. As will be explained, varying flow rates may be desired, in which case the current will be appropriately varied. Further, the frequency of excitation should be chosen to provide good depth of penetration throughout the pool to achieve appropriate fluid flow throughout all portions of the pool. The variations of these parameters will be understood by those skilled in the art.

Still further in connection with the formation of the magnetic field, although alternating current energization of varying phase has been specifically mentioned, another mode of operation is pulsed DC. The phase of pulsing will normally be varied among the different coil sections or segments, i.e., in progressively timed manner along the desired direction of flow vertically downward, in order to achieve the appropriate movement of molten metal within the pool in the ingot.

The frequency of excitation, whether A.C. or pulsed D.C., is generally of low magnitude, typically in the order of 60 hertz or less. However, it may be desirable to limit the excitation of the coils so that periodic excitation is employed. In particular, the molten metal may be made to flow in the pattern shown in FIG. 2 by continuous A.C. or pulsed D.C. excitation of the coils which is then rendered intermittent once the inertia of the moving liquid is sufficient to continue fluid flow without the use of an outside field. Selective energization of the coils is then carried out in order to keep the molten metal moving.

Still further, variation of the magnetic field turns) among the different coil sections may be employed, if desired, or the use of a coil arrangement only over a part of rather than almost the whole of the molten pool may also be employed.

In FIG. 2 a typical chill mold 10 has been shown, with the direction of ingot movement being in the vertical direction. It should be noted that ingot movement in any direction is possible and a typical chill mold need not be employed. Specifically, moving belt apparatus as disclosed in Hazelett et al., U.S. Pat. No. 3,036,348 may be utilized for ingot formation. Hence the mold 10 in FIG. 2 should be taken to represent any mold means suitable for the formation of a continuously cast structure.

As indicated above the molten metal flow rate varies the growth structure in an ingot. With forced fluid flow three basic growth structures are possible, namely, equiaxed, thamnitic or flow-modified, and fibrous.

Equiaxed dendritic structure is characterized by: (l) discernible dendrites having primary and secondary arms in which the dendrites are broken and moved to new positions by the forced fluid flow remote from the breaking positions and (2) by a random orientation of primary arms. The equiaxed dendritic microstructure is the same as free dendritic microstructure except for the fact that the breaking of dendrites is involved, and it is thus composed of broken and randomly oriented dendrites. The'equiaxed microstructure is produced by flow within the molten pool. FIGS. 3a and 3b are scanning electron microscope photographs of a solid/liquid interface of structure rendered equiaxed by laminar like flow, with the magnification x in FIG. 3a and 5001 in FIG. 3b. In the plane-section micrograph of FIG. 8, the outlines of the dendrites, with their characteristic branching configuration, are shown by the dark, sharply undulating lines or areas; the broken or separated, and considerably equiaxed structure is apparent, as well as their somewhat random orientation. It will be understood that here and in FIGS. 9 11 the very fine lines, filament-like markings or dots within the dendritic or other shapes are of no present significance; the structures herein described are those bounded by the extended or connected, dark lines or markings.

Thamnitic or flow-modified microstructure, caused by turbulent fluid flow, is an aggregate of random, round shapes. No arms whatsoever can be distinguished, primary or secondary. FIGS. 4a and 4b are scanning electron microscope photographs of the solid/liquid interface of a thamnitic or flow-modified structure produced by turbulent fluid flow. The magnification in these figures is respectively 50x and 200x. The lack of any symmetry in the thamnitic or flowmodified structure is evident. In accordance with thermodynamic considerations, dendritic growth normally proceeds into a flow, such flow being unidirectional; it is now found that where the flow becomes turbulent, lacking local directional characteristics, the solid metal can correspondingly grow in a random multitude of directions resulting in the defined thamnitic structure, the term thamnitic being intended to mean bush-like. The plane-section micrograph of FIG. 9 shows the random, closely abutting and variously connected rounded shapes of this microstructure; their appearance is substantially the same when viewed in a plane perpendicular to growth.

Fibrous microstructure is formed under intense turbulent flow. The structure is characterized by unidirectional primary structural elements only, without any secondary structural elements or secondary arms. There is an apparent braiding or interknitting of the primary structural elements by virtue of the continuously changing growth conditions in the boundary layer (in the liquid phase) because of the extremely intense turbulent flow. FIGS. 5a and 5b are scanning electron microscopephotographs of the solid/liquid interface of fibrous structure produced by turbulent flow. The magnifications are respectively 1001: and 2001:. FIGS. 10a and 10b, being plane-section micrographs taken as described above, show the elongated, somewhat criss-cross boundaries (in growth direction) and the small transverse section configurations of the somewhat fiber-like elements of this microstructure, further characterized by very little spacing between such elements.

The scanning electron microscope photographs of FIGS. 3, 4 and 5 were taken of statically cast ingots. The ingot material was A181 4335 steel. A cylindrical crucible 4 inches in diameter and 6 inches high was employed which was rotatable about a horizontal axis by The chill for the ingot was at the bottom thereof. An induction coil about the ingot provided the necessary circulation of molten metal. The well known decanting technique was used to reveal the solid/liquid interface of ingot structure. Specifically, the crucible was rotated by 180 at a given time during L la.

the circulating liquid metal through the mushy zone. Columnar growth is also inhibited by the fluid flow, since dendrites within the flow are redeposited at other points from the breaking points, assuring the compleunidirectional freezing to pour away the molten metal, tion of an equiaxed structure (random orientation of leaving the solid/liquid interface exposed for subprimary dendrite arms). Thus, although columnar sequent photographing. dendritic growth would normally be present as at point FIGS. 8 11 were also taken from ingots cast in the a in i effect of the T". fluid fl is to above crucible, the flow of molten metal being directed 0 create an gl structure the eqmvalem of across the growing solid/liquid interface, i.e., flowing F f equlvalem oftherfrfodynam' substantially parallel to the chilled surface from which sluftmg of the P a mm f growth growth proceeded. FIG 11 Shows an experiment in region of FIG. 6 IS thereby achieved by laminar-like which flow was interrupted while growth proceeded (in flow dendritic fashion) in the lefthand position of the figure, is As ncreafes turbulent 3 even then turbulent flow was effected, resulting in thamnitic though the mode of operalloil themodynamlcally growth in the central region (flow being vertical as seen mally Rresent would be the colmllllar dendrmc in the figure), and then flow was interrupted to produce region m the .copventmnal p l mgrphob' more dendritic structure at the right, the latter being gy 9 i iq f resiltmg m a I a somewhat more random and equiaxed because the termed. thamfmlc or flow-modlfied as u qui d was Somewhat charged with broken dendrites. charactierized In the third column of the above table as random round shapes. No arm structures are really These three new basic growth structures are discernible in the thamnitic structure. described in the following table which compares such At still higher velocities of molten metal, the local structures to the undesirable columnar dendritic strucgrowth within the ingot proceeds generally in one ture which is present without any forced fluid flow durdirection. Such growth is characterized herein as ing the casting of an ingot. The desirable characteristics fibrous and is described in the fourth column of Table of the equiaxed dendritic structure, the thamnitic or Iabove. Such growth structure is equivalent to cellular flow-modified structure, and the fibrous structure are growth under equivalent thermodynamic conditions apparent from the table. with no fluid flow.

TABLE I Columnar Dendritic Equlaxed dendritic structure Tharnnitic or flow modified Fibrous structure structure (no flow) (caused by flow) structure (turbulent flow) (turbulent flow) Physical Unidircctionally aligned Random accumulation and Random round shapes Unidirectionally aligned characteristics. dendrites (primary arms orientation of broken formed in situ. primary arms only, which aligned). dendrites. interkmt or braid among themselves. Strength roperties Antisotropic (strongest in Isotropic Isotropic the growth direction). Ductility Poor across growth direction Good Good Good Nature of mushy Appreciable Reduced Very small Practically none.

zone. 1n 1nsions Present; size and volume Present; but of fairly Few present; fairly constant Present; constant nd ll iractopm increased inconstant size and volume and small size and volume small size, small volume wardly from the chill. fraction. fraction (volume fraction fraction decreasing may decrease slightly inwardly of chill. Rllawfllgly of the chill). s b n d d Mierose re ation Present Pr nt 8 I10? u stantia y re uce Macrose gre gationnn do Reduced substantially Very slight or none Non FIG. 6 is a graphical representation of the various growth regimes with no flow as a function of solute concentration C and the critical ratio G/R, in which G is the temperature gradient in the molten pool at the solid/liquid interface and R is the growth rate of solid material in a direction perpendicular to the solid/liquid interface. There are four major categories of growth in FIG. 6, namely: (a) free dendritic growth (no particular orientation of primary dendrite arms); (b) columnar dendritic growth (alignment of primary dendrite arms); (c) cellular growth (primary but no secondary dendrite arms); and (d) planar growth (no dendrite structure).

The present invention operates with a liquid pool within an ingot in which the factors of solute concentration and critical ratio would normally thermodynamically result in columnar dendritic growth, e.g., as at point a in FIG. 6. With laminar like flow, fragmentation or breaking of dendrites takes place so as to reduce the width of the columnar zone that would normally be present by at least two mechanisms: dynamic shearing of dendrites and the dissolution of dendrite stems by it will be noted then that the flow rate determines the ingot microstructure and, depending upon the type of structure desired, the flow rate is chosen accordingly.

FIG. 7 is a graphical representation of the flow speed required for the three individual and different microstructures described above, namely, equiaxed (at least partially equiaxed), flow-modified or thamnitic, and fibrous, as a function of an initial temperature. It will be noted that the flow rate is directly correlated with the power input to the coils that cause the molten metal flow, and hence the ordinate in FIG. 7 is characterized as flow rate or power input to stirring coils. In FIG. 7 the equiaxed regime is the area under curve 40. The flow-modified or thamnitic regime is the area between curves 40 and 42. The fibrous regime is the area above curve 42.

Investigations of cast-in-place ingots with induced fluid flow have been completed on AISI 4335 type steel. The following has been concluded: (the figures given below represent average flow speeds and refer to a melt with an initial temperature of 2,850 F.):

1. induced fluid flow of low velocity (below 20 cm/sec) produces laminar-like flow conditions that change a columnar dendritic structure to an equiaxed dendritic structure at higher thermal gradients or shorter distances from an established chill than obtained under static solidification conditions. As the velocity of the induced flow increases (up to 25 cm/sec) the amount of fragmentation of the columnar dendritic structure increases in unidirectionally solidified ingots until this columnar structure is completely eliminated. At still higher velocities (above 25 cm/sec) the induced flow becomes turbulent and the morphology of the solidified structures becomes flow-modified or thamnitic with local growth proceeding in multidirections. Turbulent fluid flow of very high velocity (above 55 cm/sec) produces a fibrous structure with an appearance similar to a cellular structure.

2. Increasing flow velocity decreases the growth rate of the unidirectionally growing solid and increases the temperature gradient at the solid-liquid interface by a combined mechanical and thermal effect. This effect of flow is more marked at increased distances from the chill.

3. Turbulent fluid flow past the growing solid minimizes the width of the mushy zone, eliminates macrosegregation and reduces the severity of microsegregation.

4. Turbulent flow of at least the velocity necessary for the formation of flow-modified or thamnitic structure prevents the coarsening of inclusions that usually occurs as the distance from the chill increases; at the higher flow velocities which form fibrous structure, the inclusion volume fraction in the solid decreases as the distance from the chill increases.

5. The tensile properties of the flow-modified structure are similar to the statically cast columnar dendritic close to the chill, but do not exhibit the loss in ductility (mass effect) at greater distances from the chill of the statically solidified structure.

6. The fibrous structure is similar in tensile properties to the columnar and flow-modified or thamnitic structure.

7. The interdendritic distances are much less in the equiaxed dendritic structure than they are in the columnar dendritic structure, providing less space for segregates and inclusions to be trapped in the microstructure. This spacing becomes less between the elements of growth in the thamnitic microstructure and very much less in the fibrous microstructure.

It may be specifically explained that in normal solidification of metal, with little or no flow, there is a so-called mushy zone of relatively substantial extent between the growing solid and the truly liquid metal. In this zone, segregation occurs involving localized increased concentration of solutes, and is manifested in the product, both on a microscale (as defined above), and on a macro-scale by a resulting continuity of the increased solute concentration over a considerable area or distance. Moreover, formation of inclusions, i.e., socalled indigenous inclusions, is greatly promoted in the mushy zone as to atoms of elements such as oxygen and sulfur which there become concentrated in the liquid phase, and combine with metallic elements in the mushy zone. With substantial fluid flow, in the present invention, occurrence of segregation and formation of inclusions are greatly reduced, as has been stated above. The extent or thickness of the mushy zone is reduced, indeed essentially to none in the case of the intense flow that creates a fibrous structure, and the motion of the metal carries away the localized portions of high solute concentration and the localized content of inclusion-forming elements, so that the latter are dissolved and the former redistributed in normal intended concentration in the liquid. The reduction of the mushy zone is aided or characterized by the reduction of spacing between growing solid structures, such as dendrites, thus cooperating in the elimination of segregation and inclusion problems. Finally, the thermodynamic conditions between solid and liquid phases are altered by the movement of liquid and its proximity to the solid phase, in a direction toward greater equality of solute concentration in adjacent solid and liquid regions, as is most desirable for avoidance of segregation in the ultimate cast body.

An advantage of the forced flow of molten metal in the pool inside a continuously cast ingot is that segregates and inclusions tend to be confined to the pool. The result is that at the end of a casting run, the pool is rich in solutes and segregates. As noted above, the end of the ingot which is last cast and which is unusable is shorter than that involved when no forced flow of fluid is provided.

In further summary of the several results described hereinabove, the laminar or turbulent metal flow sweeping along the solid/liquid interface carries away elements of the mushy zone, and in particular tends to carry away solute remaining between growth elements, thus counteracting segregation and reducing or preventing formation of inclusions (from segregated inclusion elements) at the locality of solidification. Indeed the reduced spacing between growing solid structures, as mentioned above, necessarily limits the size of inclusions even if left to grow in such spaces. To the extent that inclusions are already present in the molten metal or form in the flow or pool, the return flow preferably carries such inclusions back up to the underside of the slag layer. As explained and shown, the path of circulating flow is preferably carried up into the mold beneath the slag layer, and then is initiated downwardly along the solidifying metal, being advanced effectively along such interface below the mold by the described nature of magnetic field. Moreover, as set forth in Table I above, the employment of turbulent flow (greatly reducing the spacing between growing structures) is markedly effective in obviating occurrence of unwanted inclusions in the product, and likewise in reducing or obviating both microand macro-segregation.

It will be understood that although some illustrative tests herein have been effected with a selected, representative, low alloy steel, the invention is applicable to casting of metals in a general sense, including all varieties of steel, whether so-called ordinary carbon grades, or various low or rich alloy steels, or stainless or other special grades. The improvements in procedure and products are basically concerned with structures, i.e., microstructures, and improved physical effects occuring or produced in the course of solidification of molten metal, and although the invention is specially concerned with continuous casting procedure of the sort described above, the underlying principles can be deemed applicable to casting in a general sense. A particular advantage, emphasized above and of notable value in continuous casting, is the production of unusually clean, uniform metal, in a reliable and readily controlled manner.

By way of further and specific example of a coil unit disposed around the descending strand (i.e., ingot) of a continuous casting operation, one such coil unit consisted of three IO-turn sections of copper tubing, carrying cooling water, arranged in continuing immediate succession along the strand path, the turns being square with rounded comers and positioned helically around the square-section strand. in one instance of use with apparatus casting steel, e.g., an 8 inch by ten inch ingot at a rate of ingot descent of 44 inches per minute downward through a chill mold having a height of 3 feet, the sections were respectively connected to the phases of a conventional three-phase A.C. current source, the phase difference (120) being progressive down the unit, in such order as to effect molten metal flow downward (from a region above to a region below the unit) along the solid/liquid interface and back up the center of the liquid metal pool. It will be understood that the speed of downward flow induced in the molten metal in all cases is faster than the downward speed of the ingot and its pool as a whole, e.g., of different, higher order of velocity.

With each section of the unit carrying about 800 amperes at 500 volts, the flow of molten metal appeared to be sufficiently rapid to produce a thamnitic structure in a band of inwardly growing solidification that occurred as the strand passed the coil unit, with some favorable effect on microstructure at localities of solidification further inward. Indeed some utility was achieved with a single such coil unit, 30 inches tall (10 inches per section) disposed with its upper end 5 feet below the bottom of the mold: the microstructure was somewhat improved at least at regions inward of about 1% inches from the strand periphery, and centerline shrinkages or cavities were reduced or minimized in the overall sense of providing a better cast product than in many cases previously obtained or obtained without an undesirably lower temperature of metal supplied in the tundish.

Special and notably improved results of the invention are attained, however, where the electromagnetic influence of the coils covers at least a substantially longer portion of the molten pool (which may reach 35 feet or more below the mold), indeed preferably most or all of it; for instance, another like coil unit, in the above apparatus, can be effectively interposed between the mold and the unit located as above, and preferably one or more further such units disposed below the latter along the descending strand.

The invention should be taken to be defined by the following claims.

We claim:

1. In the continuous casting of an. ingot in which there is present within the ingot a pool of molten metal which is surrounded by at least a solidified zone of metal and which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the subsequent solidification structure of the molten metal, comprising causing a circulating flow of said molten metal within said pool that:

a. continuously sweeps at a velocity which provides turbulent flow along at least a selected length of a solid/liquid interface posed between said pool and solidified zone to define a first direction toward one end of the pool, said circulating flow being electromagnetically effected by supply of electrical power sufficient to produce said turbulent flow velocity, and

b. returns in the interior of the pool in a direction opposite to said first direction to complete said circulation.

2. A method according to claim 1, in which the first direction of metal flow is the same as the direction of ingot movement and sweeps along said interface downstream from said mold means, and in which said power is supplied to effectuate said flow along said interface at a velocity above 25 cm/sec.

3. A method according to claim 2, in which the solute concentration C, within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth, the rate of flow of molten metal being between 25 and 55 cm/sec. and of sufficient magnitude so as to produce thamnitic or flow-modified structure within the ingot.

4. A method according to claim 2, in which the solute concentration C, within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth in the ingot, the rate of flow of molten metal being above 55 cm/sec. and sufficient to result in fibrous structure within the ingot.

5. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the solidification structure of the ingot comprising causing a circulating flow of molten metal within said pool that is:

a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface downstream from said mold means, and

. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool;

c. said flow of molten metal being produced by a moving magnetic field downstream of said mold means, said moving magnetic field being produced by polyphase excitation of successive coil sections which are arranged along and coaxial with the path of ingot movement, and of which each section is a coil helically coaxial with said path.

6. A method according to claim 5, in which the flow of molten metal is in turbulent flow along said interface, produced by supplying sufficient electrical power to said coil sections to provide turbulent flow velocity of metal.

7. A method according to claim 6, in which the turbulent flow of molten metal is produced at a velocity above 25 cm/sec.

8. A method according to claim 6, in which the solute concentration C, within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth, the rate of flow of molten metal being turbulent and of sufficient magnitude so as to produce thamnitic or flow-modified structure within the ingot.

9. A method according to claim 6, in which the solute concentration C, within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth in the ingot, the rate of flow of molten metal being intensely turbulent and sufficient to result in fibrous structure within the ingot.

10. A method according to claim 6, in which in the region of said mold means the flow of molten metal is at least partially along the underside of the slag layer that is created.

11. A method according to claim 6, in which said field extends substantially along the entire length of the ingot as far as said pool extends within the ingot.

12. A method according to claim 2, in which molten metal is initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer.

13. A method according to claim 12, in which the introduced metal is directed generally toward the region 1 of the intersection of the slag layer with the inner walls of said mold means.

14. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which extends downstream at least about 30 feet from the location of about 2 to 3 feet in length at which the outer skin of the ingot is initially formed, the method for improving the solidification structure of the ingot comprising: causing, by the application of force created by a moving magnetic field extending downwardly throughout a zone along the ingot for at least a 30-foot length of said pool below about the first 2 feet thereof that comprise said location at which the outer skin of the ingot is formed, a circulating flow of molten metal within said pool that is:

a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface for substantially the full extent thereof, and

hr in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool,

c. said moving magnetic field being produced by polyphase excitation of successive coil sections which are arranged along and coaxial with the path of ingot movement, and of which each section is a coil helically coaxial with said path,

said flow causing a change in the cast structure from that which would normally be present given the particular solute concentration C within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material i a iiirection transverse to said iqterface.

n the continuous casting 0 an ingot in which there is present within the ingot a pool of molten metal which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the solidification structure of the ingot comprising producing, by a moving magnetic field downstream of said mold means, a circulating flow of molten metal within said pool that is:

a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface downstream from said mold means, and

. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool;

c. the molten metal being initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer, said directing of the introduced metal and the aforesaid flow-producing action of the magnetic field downstream of the mold means coacting to carry said circulating flow along a path within said mold means whereby the return direction flow extends centrally up to and outwardly under the slag and thence the first direction flow along the interface extends downwardly within the mold means,

. said first direction flow of molten metal below the mold means being turbulent flow, effected by producing said moving magnetic field with sufficient electrical power to provide turbulent flow velocity of above 25 cm/sec.

. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool;

c. the molten metal being initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer, said directing of the introduced metal and the aforesaid flow-producing action of the magnetic field downstream of the mold means coacting to carry said circulating flow along a path within said mold means whereby the return direction flow extends centrally up to and outwardly under the slag and thence the first direction flow along the interface extends downwardly within the mold means,

. said first direction flow of molten metal below the mold means being turbulent flow, effected by producing said moving magnetic field with sufficient electrical power to provide turbulent flow velocity of above 25 cm/sec.

UNITED STATES PATENT OFFICE CERTIFICATE OF CURRECTION Patent 3,693,697 Dated September 26, 1972 Alexander-A. Tzavaras' Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Abstract, line 19 after "thamnitic", "of" should be --or-- Col 7 line 38 after "fie1d" insert (ampere Col. 9 in Table second column (line 11) for "fractopm tead -fraction- Col. 16, cancel lines 39-59.

Signed and sealed this 27th'day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. 1 ROBERT GOTTSCHALK Att'esting Officer Commissioner of Patents U 5. GOVERNMENT PRINTING OFFICE I959 Q-3b6334 

1. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which is surrounded by at least a solidified zone of metal and which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the subsequent solidification structure of the molten metal, comprising causing a circulating flow of said molten metal within said pool that: a. continuously sweeps at a velocity which provides turbulent flow along at least a selected length of a solid/liquid interface posed between said pool and solidified zone to define a first direction toward one end of the pool, said circulating flow being electromagnetically effected by supply of electrical power sufficient to produce said turbulent flow velocity, and b. returns in the interior of the pool in a direction opposite to said first direction to complete said circulation.
 2. A method according to claim 1, in which the first direction of metal flow is the same as the direction of ingot movement and sweeps along said interface downstream from said mold means, and in which said power is supplied to effectuate said flow along said interface at a velocity above 25 cm/sec.
 3. A method according to claim 2, in which the solute concentration Co within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth, the rate of flow of molten metal being between 25 and 55 cm/sec. and of sufficient magnitude so as to produce thamnitic or flow-modified structure within the ingot.
 4. A method according to claim 2, in which the solute concentration Co within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth in the ingot, the rate of flow of molten metal being above 55 cm/sec. and sufficient to result in fibrous structure within the ingot.
 5. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the solidification structure of the ingot comprising causing a circulating flow of molten metal within said pool that is: a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface downstream from said mold means, and b. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool; c. said flow of molten metal being produced by a moving magnetic field downstream of said mold means, said moving magnetic field being produced by polyphase excitation of successive coil sections which are arranged along and coaxial with the path of ingot movement, and of which each section is a coil helically coaxial with said path.
 6. A method according to claim 5, in which the flow of molten metal is in turbulent flow along said interface, produced by supplying sUfficient electrical power to said coil sections to provide turbulent flow velocity of metal.
 7. A method according to claim 6, in which the turbulent flow of molten metal is produced at a velocity above 25 cm/sec.
 8. A method according to claim 6, in which the solute concentration Co within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth, the rate of flow of molten metal being turbulent and of sufficient magnitude so as to produce thamnitic or flow-modified structure within the ingot.
 9. A method according to claim 6, in which the solute concentration Co within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface are such as to thermodynamically normally result in columnar dendritic growth in the ingot, the rate of flow of molten metal being intensely turbulent and sufficient to result in fibrous structure within the ingot.
 10. A method according to claim 6, in which in the region of said mold means the flow of molten metal is at least partially along the underside of the slag layer that is created.
 11. A method according to claim 6, in which said field extends substantially along the entire length of the ingot as far as said pool extends within the ingot.
 12. A method according to claim 2, in which molten metal is initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer.
 13. A method according to claim 12, in which the introduced metal is directed generally toward the region of the intersection of the slag layer with the inner walls of said mold means.
 14. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which extends downstream at least about 30 feet from the location of about 2 to 3 feet in length at which the outer skin of the ingot is initially formed, the method for improving the solidification structure of the ingot comprising: causing, by the application of force created by a moving magnetic field extending downwardly throughout a zone along the ingot for at least a 30-foot length of said pool below about the first 2 feet thereof that comprise said location at which the outer skin of the ingot is formed, a circulating flow of molten metal within said pool that is: a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface for substantially the full extent thereof, and b. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool, c. said moving magnetic field being produced by polyphase excitation of successive coil sections which are arranged along and coaxial with the path of ingot movement, and of which each section is a coil helically coaxial with said path, said flow causing a change in the cast structure from that which would normally be present given the particular solute concentration Co within said pool and the temperature gradient G at said interface at the liquid side thereof and the growth rate R of solid ingot material in a direction transverse to said interface.
 15. In the continuous casting of an ingot in which there is present within the ingot a pool of molten metal which extends downstream a substantial distance from the mold means within which the outer skin of the ingot is formed, the method for improving the solidification structure of the ingot comprising producing, by a moving magnetic field downstream of said mold means, a circulating flow of molten metal withiN said pool that is: a. along the solid/liquid interface of the ingot in a first direction which is the same as the direction of ingot movement and which sweeps along said interface downstream from said mold means, and b. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool; c. the molten metal being initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer, said directing of the introduced metal and the aforesaid flow-producing action of the magnetic field downstream of the mold means coacting to carry said circulating flow along a path within said mold means whereby the return direction flow extends centrally up to and outwardly under the slag and thence the first direction flow along the interface extends downwardly within the mold means, d. said first direction flow of molten metal below the mold means being turbulent flow, effected by producing said moving magnetic field with sufficient electrical power to provide turbulent flow velocity of above 25 cm/sec. b. in a return direction opposite to the first direction within a longitudinal zone in the interior of said pool; c. the molten metal being initially supplied to said mold means from a central zone underneath the slag layer, with the introduced metal being directed outwardly and generally toward the underside of the slag layer, said directing of the introduced metal and the aforesaid flow-producing action of the magnetic field downstream of the mold means coacting to carry said circulating flow along a path within said mold means whereby the return direction flow extends centrally up to and outwardly under the slag and thence the first direction flow along the interface extends downwardly within the mold means, d. said first direction flow of molten metal below the mold means being turbulent flow, effected by producing said moving magnetic field with sufficient electrical power to provide turbulent flow velocity of above 25 cm/sec. 